The present invention relates generally to magnetic transducers for reading information signals recorded in a magnetic medium and, more particularly, to a magnetoresistive read sensor based on the giant magnetoresistance exhibited by multiple layers of individual ferromagnetic particles fixed in a matrix of nonmagnetic conductive material.
It is well-known in the prior art to utilize a magnetic read transducer referred to as a magnetoresistive (MR) sensor or head for reading high density recorded data from magnetic media. An MR sensor detects magnetic field signals through the resistance changes of a read element fabricated of a magnetic material as a function of the strength and direction of magnetic flux being sensed by the read element. These prior art MR sensors operate on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine (cos.sup.2) of the angle between the magnetization and the direction of sense current flow through the read element. A more detailed description of the AMR effect can be found in "Memory, Storage, and Related Applications", D. C. Thompson et al., IEEE Trans. Mag. MAG-11, p. 1039 (1975).
U.S. Pat. No. 4,896,235 entitled "Magnetic Transducer Head Utilizing Magnetoresistance Effect", granted to Takino et al on Jan. 23, 1990, discloses a multilayered magnetic sensor which utilizes the AMR effect and comprises first and second magnetic layers separated by a non-magnetic layer and in which at least one of the magnetic layers is of a material exhibiting the AMR effect. The easy axis of magnetization in each of the magnetic layers is set perpendicular to the applied magnetic signal such that the MR sensor element sense current provides a magnetic field in the magnetic layers parallel to the easy axis thus eliminating or minimizing Barkhausen noise in the sensor. "Thin Film MR Head for High Density Rigid Disk Drive"by H. Suyama et al, IEEE Trans. Man., Vol. 24, No. 6, 1988 (pages 2612-2614) discloses an multilayered MR sensor similar to that disclosed by Takino et al.
A second, different and more pronounced magnetoresistive effect has also been described in which the change in resistance of a layered magnetic sensor is attributed to the spin-dependent transmission of conduction electrons between ferromagnetic layers via a nonmagnetic layer separating the ferromagnetic layers and the accompanying spin-dependent scattering at the layer interfaces. This magnetoresistive effect is variously referred to as the "giant magnetoresistive" or "spin valve" effect. Such a magnetoresistive sensor fabricated of the appropriate materials provides improved sensitivity and greater change in resistance than observed in sensors utilizing the AMR effect. In this type of MR sensor, the in-plane resistance between a pair of ferromagnetic layers separated by a non-magnetic layer varies as the cosine (cos) of the angle between the magnetization in the two layers.
U.S. Pat. No. 4,949,039 to Grunberg describes a layered magnetic structure which yields enhanced MR effects caused by antiparallel alignment of the magnetization in the magnetic layers. As possible materials for use in the layered structure, Grunberg lists ferromagnetic transition metals and alloys, but does not indicate preferred materials from the list for superior MR signal amplitude. Grunberg further describes the use of antiferromagnetic-type exchange coupling to obtain the antiparallel alignment in which adjacent layers of ferromagnetic materials are separated by a thin interlayer of Cr or Y.
Co-pending U.S. Patent application Ser. No. 07/625,343 filed Dec. 11, 1990 now U.S. Pat. No. 5,206,590, assigned to the instant assignee, discloses an MR sensor in which the resistance between two uncoupled ferromagnetic layers is observed to vary as the cosine of the angle between the magnetizations of the two layers and which is independent of the direction of current flow through the sensor. This mechanism produces a magnetoresistance that is based on the spin valve effect and, for selected combinations of materials, is greater in magnitude than the AMR.
U.S. Pat. No. 5,159,513 granted to Dieny et al on Oct. 27, 1992, assigned to the instant assignee, discloses an MR sensor based on the above-described spin valve effect which includes two thin film layers of ferromagnetic material separated by a thin film layer of anon-magnetic metallic material wherein at least one of the ferromagnetic layers is of cobalt or a cobalt alloy. The magnetization of the one ferromagnetic layer is maintained perpendicular to the magnetization of the other ferromagnetic layer at zero externally applied magnetic field by exchange coupling to an antiferromagnetic layer.
The spin valve structures described in the above-cited U.S. Patents require that the direction of magnetization in one of the two ferromagnetic layers be fixed or "pinned" in a selected orientation such that under non-signal conditions the direction of magnetization in the other ferromagnetic layer is oriented perpendicular to the pinned layer magnetization. Additionally, in both the AMR and spin valve structures, in order to minimize Barkhausen noise, it is necessary to provide a longitudinal bias field to maintain at least the sensing portion of the read element in a single magnetic domain state. Thus, a means for both fixing the direction of the magnetization and providing a longitudinal bias field is required. For example, as described in the above-cited patent application and patents, an additional layer of antiferromagnetic material can be formed in contact with the ferromagnetic layer to provide an exchange-coupled bias field. Alternatively, an adjacent magnetically hard layer can be utilized to provide hard bias for the ferromagnetic layer.
More recently, the giant magnetoresistance effect has been observed in both single- and multi-layer heterogeneous films of ferromagnetic particles embedded in a nonmagnetic metallic matrix. GMR has been reported in inhomogeneous, single layer alloy systems such as cobalt-copper (Co-Cu), cobalt silver (Co-Ag) and nickel-iron-silver (NiFe-Ag), For example, see "GIANT MAGNETORESISTANCE IN NONMULTILAYER MAGNETIC SYSTEMS", John Q. Xiao et al, PHYSICAL REVIEW LETTERS, Vol. 68, No. 25, pages 3749-3752 (Jun. 22, 1992); "GIANT MAGNETORESISTANCE IN HETEROGENEOUS CU-CO ALLOYS", A. E. Berkowitz et al, PHYSICAL REVIEW LETTERS, Vol. 68, No. 25, pages 3745-3748 (Jun. 22, 1992); "`GIANT` MAGNETORESISTANCE OBSERVED IN SINGLE LAYER CO-AG ALLOY FILMS", J. A. Barnard et al, Letter to the Editor, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 114 (1992), pages L230-L234; and J. Jaing et al, APPLIED PHYSICS LETTERS, Vol. 61, page 2362 (1992). The Co alloys are of materials that are immiscible at low temperatures. However, annealing the metastable alloy causes the formation of fine Co precipitates, i.e., "grains", in a Cu or Ag matrix. The MR effect appears to vary inversely with the diameter of the average particle diameter. While the "granular" alloy is an attractive approach to achieve a GMR sensor with only a single film in contrast to the more complex multilayered spin valve sensors, the Co alloys require saturation fields of several thousand Oersted (Oe) and would not be useful in a low field MR sensor application. Recently reported work with granular NiFe in a Ag matrix has yielded MR ((delta R)/R) of 10 percent at room temperature with the half-height width of the MR peak of about 400 Oe. More recent work with annealed NiFe multilayers in a Ag matrix has yielded a granular multilayer structure which exhibits an MR of approximately 17 percent at room temperature with a saturation field of about 100 Oe. See "MAGNETORESISTIVE PROPERTIES AND THERMAL STABILITY OF NI-FE/AG MULTILAYERS", B. Rodmacq et al, Letter to the Editor, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 118 (1993), pages L11-L16. Although the saturation fields for the single- and multiple-layer NiFe alloys are lower than the reported value for Co alloys, it is still above the range that is useful for MR devices (about 10 Oe or less).